BOSTON, Oct. 18 - A canon lawyer seeking to have Senator John Kerry excommunicated by the Roman Catholic Church because of his support for abortion rights said on Monday that he had ammunition in the form of a letter issued at the request of a senior Vatican official.

The lawyer, Marc Balestrieri of Los Angeles, who heads a conservative Catholic nonprofit organization called De Fide, also said that, based on the letter, he would now seek to have four other Catholic politicians excommunicated: Senators Edward M. Kennedy of Massachusetts, Tom Harkin of Iowa, Susan Collins of Maine, and Mario M. Cuomo, the former governor of New York.

"Senator Kerry, and all pro-choice Catholic politicians, who publicly call themselves Catholic yet who blatantly violate canon law by continuing to profess heresy and receive Holy Communion, must publicly reject their abortion advocacy for the sake of their own souls, and the others they have scandalized," Mr. Balestrieri said in a statement. "They have been excommunicated."

Only Ms. Collins is not a Democrat.

The letter to Mr. Balestrieri, written by another American canon lawyer at the request of a Vatican official, says that "if a Catholic publicly and obstinately supports the civil right to abortion, knowing that the church teaches officially against that legislation, he or she commits that heresy" and is "automatically excommunicated."

The letter, first reported on Friday by Eternal Word Television Network, a Catholic station in Alabama, was written last month after Mr. Balestrieri met with an official at the Vatican's Congregation for the Doctrine of the Faith.

In June, Mr. Balestrieri filed a complaint accusing Mr. Kerry of heresy and seeking to have him excommunicated. The complaint was filed with the Archdiocese of Boston, where Mr. Kerry lives. The archdiocese, which declined to comment Monday, can decide whether Mr. Kerry should be excommunicated because he supports abortion rights.

Mr. Balestrieri said that when he was in Rome, he did not disclose to the Vatican that he had filed the petition against Mr. Kerry. He said he merely asked whether someone who publicly supports abortion rights was guilty of heresy.

Experts in canon law say Vatican officials frequently receive questions about matters of church doctrine. They can choose to answer those questions officially, giving their answers the weight of church law, or they can choose to answer them unofficially, delegating the matter to a canon lawyer not affiliated with the Vatican.

In Mr. Balestrieri's case, the undersecretary of the office, the Very Rev. Augustine di Noia, asked a friend, the Rev. Basil Cole, an associate professor at the Dominican House of Studies in Washington, to write an "unofficial" response to Mr. Balestrieri's question, Father Cole said in an interview.

A spokesman for Mr. Kerry, Michael Meehan, would say only: "Kerry's a Catholic who attends Mass regularly and receives communion. That explains his standing in the church."

A spokesman for Mr. Kennedy declined to comment. Mr. Harkin, Ms. Collins and Mr. Cuomo could not be reached.